To be honest, they may have looked at your feedback profile and the feedback that you've left for other sellers and decided that they didn't want to risk the same, so cancelled the order.
Plus, you mentioned in the feedback that "posted a message that I only had £3.51 plus postage until the end of the month," and it's now only mid month.
You've left a negative, so that's the most you can do here to damage their account.